There is sometimes a stigma that sperm donors are undesirable men, looking to make a quick buck. The truth is that sperm donation is extremely selective. Scott Brown, director of client experience at California Cryobank, one of the largest frozen-tissue banks in the world, estimates that 99 percent of potential donors are screened out of California Cryobank's selection process, and he says other banks are similarly picky. One immediate barrier is that all California Cryobank donors must have a degree from or be enrolled in a four-year college.

As expected, sperm quality, including how much there is, how well it moves (motility), and how well it's shaped (morphology), matters to sperm banks. But this is a much bigger deal than most men realize. "Most guys assume they must be great and have great sperm and, therefore, can be a donor just by saying they want to do it," says Brown, "and the reality is it's actually very difficult to qualify to be a sperm donor." Even if a man is perfectly capable of fathering a child on his own, he might not meet the very high standards of the sperm bank.

3. Your personality sucks.

"If a donor is a difficult person to work with or doesn't seem honest or [our coordinator is] made uncomfortable by the guy, they get eliminated," says Brown. In other words, California Cryobank is allowed to disqualify potential donors even if they don't seem like high-quality candidates by purely subjective measures.

Following insights from supply and demand, California Cryobank doesn't take on donors who have a BMI outside the normal range. Among white donors, they also require their donors to be taller than 5'10". For other ethnicities, including Hispanic and Asian, the height requirement is less stringent because there are fewer applicants. Brown has heard of additional limitations from other banks, including no redheads. California Cryobank doesn’t have that rule because they want a diverse donor pool. "You want to have as complete a catalog as possible and have as many options [as possible] for people," Brown says. They usually have about 525 donors to choose from at any time.

5. Your genes don't pass the Gattaca test.

Donors will undergo genetic testing to check for mutations. Although everyone harbors genetic mutations and these are often harmless, if a donor's sperm contains the same mutation as the egg it fertilizes, that puts the offspring at risk of having that disease. Some genes California Cryobank looks at include those that could cause cystic fibrosis, spinal muscular atrophy, thalassemia, and Tay-Sachs.

It takes three months to get through the application process at California Cryobank and nine months to get added to the catalog. How long a man donates depends on how often they come in. The bank allows for 25 to 30 family units per donor, but Brown says most men hit about 15.

7. You hate homework.

In addition to asking family about medical histories, there are many other assignments California Cryobank gives its donors. "There are written pieces of information — essays and short answer questions, profiles — that they have to fill out," says Brown. This includes information about educational background, awards, work history, aspirations, and likes and dislikes. Donors also have to supply photos of themselves from childhood, sometime between the ages of two and around 11. They also make an audio recording. All of this information is made available to the recipients of the sperm in hard copy (and a CD) once a pregnancy is confirmed.

8. You aren't ready to have children in 2222.

Donations can be used as long as they are properly kept. “As long as it's stored in liquid nitrogen and isn’t allowed to thaw at all, they're pretty much [viable] forever,” says Brown. This means a donor could have offspring many years after he donates. In 2009, a couple reportedly had a baby using sperm that had been frozen for 22 years, according to ABC News. (This was not donor sperm but rather the sperm of the legal father of the baby that was frozen for medical reasons.)